The pikes were housed separately, and researchers collected water from each tank to make test mixtures.

In one case, minnows were exposed to two different batches of pike-scented water.

The specimens contained equal volumes of water (60 milliliters), but one batch was made with the scents of two pikes, the other with those of twelve.

The minnows reacted more strongly to the two-pike mixture, because it contained more chemicals-per-pike.

This indicated that the minnows were using the intensity of the smell to gauge how far away each pike was, since a concentrated pike smell in the wild would mean pikes were nearby.

Next, the researchers exposed minnows to either 60 ml of pike essence made from 12 pikes, or 10 ml made from two pikes.

The concentration of pike chemicals was the same in each batch: 5 ml per pike.

This time, the minnows reacted more strongly to the 12-pike mixture than the two-pike one, proving they could distinguish between individual pikes.

Classically conditioned

All the minnows used in the experiment were collected from lakes that contained pikes.

This was important because minnows, like many fish, don't have an innate knowledge of which predators to avoid when they are born.

It's something the fish have to learn, either by trial and error, such as a near-escape from a predator fish, or by example.

When the skin of a minnow is torn open, unique chemicals are released that put other minnows on high alert.

Minnows mark other fish as dangerous using a type of learning called classical conditioning.

The Russian behaviorist Ivan Pavlov first discovered classical conditioning when he famously trained dogs to associate the sound of a bell with food. After a few trials, the dogs would salivate to the sound of the bell even when no food was given.

In the case of minnows, they learn to associate both the smell of broken minnow-flesh and the odor of predator pikes with danger.

But whereas Pavlov's dogs required numerous trials before the connection between bell and food finally clicked, minnows require only a single predator exposure to learn the lesson for life.

"If you want to survive, the selection pressure to learn as efficiently as possible is right there, and it's very high," study team member Maud Ferrari told LiveScience.

"Maybe some animals don't [learn] and those animals don't survive and reproduce," she added.